Incremental HTTP Messages

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Table of Contents

1. Introduction

HTTP [HTTP] permits receivers to begin processing portions of HTTP messages as they arrive, rather than requiring them to wait for the entire HTTP message to be received before acting.¶

Some applications are specifically designed to take advantage of this capability.¶

For example, Server-Sent Events [SSE] uses a long-running HTTP response, where the server continually sends notifications as they become available.¶

In the case of Chunked Oblivious HTTP Messages [CHUNKED-OHTTP], the client opens an HTTP request and incrementally sends application messages, while the server can start responding even before the HTTP request is fully complete. In this way, the HTTP request-response pair could create what is, in effect, a bi-directional communication channel.¶

Applications that rely on incremental delivery of messages are fragile when HTTP intermediaries are involved. This is because HTTP intermediaries are not only permitted but are frequently deployed to buffer complete HTTP messages before forwarding them downstream (Section 7.6 of [HTTP]).¶

If such a buffering HTTP intermediary exists between the client and the server, these applications may fail to function as intended.¶

In the case of Server-Sent Events, an intermediary that tries to buffer the HTTP response completely before forwarding it could be left waiting indefinitely. A client might never receive any portion of the response.¶

In the case of requests that involve any bi-directional exchange, an intermediary that tries to buffer entire messages -- either request or response -- prevents any data from being delivered.¶

To help avoid such behavior, this document specifies the "Incremental" HTTP header field, which instructs HTTP intermediaries to begin forwarding the HTTP message downstream before receiving the complete message.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document relies on structured field definitions of Item and Boolean [STRUCTURED-FIELDS].¶

3. The Incremental Header Field

The Incremental HTTP header field expresses the sender's intent for HTTP intermediaries to start forwarding the message downstream before the entire message is received.¶

The Incremental header field is defined as a structured field [STRUCTURED-FIELDS] of type Item. There is just one valid value, which is of type Boolean: "?1".¶

Incremental = ?1

Upon receiving a header section that includes an Incremental header field with a true value, HTTP intermediaries SHOULD NOT buffer the entire message before forwarding it. Instead, intermediaries SHOULD transmit the header section downstream and continuously forward the bytes of the message body as they arrive. As the Incremental header field indicates only how the message content is to be forwarded, intermediaries can still buffer the entire header and trailer sections of the message before forwarding them downstream.¶

The Incremental HTTP header field applies to each HTTP message. Therefore, if both the HTTP request and response need to be forwarded incrementally, the Incremental HTTP header field MUST be set for both the HTTP request and the response.¶

The Incremental field is advisory. Intermediaries that are unaware of the field or that do not support the field might buffer messages, even when explicitly requested otherwise. Clients and servers therefore cannot expect all intermediaries to understand and respect a request to deliver messages incrementally. Clients can rely on prior knowledge or probe for support on individual resources.¶

The Incremental header field facilitates the establishment of a bidirectional byte channel over HTTP, as its presence in both requests and responses instructs intermediaries to forward early responses (Section 7.5 of [HTTP]) and to transmit message contents incrementally in both directions. However, when developing bidirectional protocols over HTTP, Extended CONNECT [RFC8441][RFC9220] is generally more consistent with HTTP's architecture.¶

4. Security Considerations

To conserve resources required to handle HTTP requests or connections, it is common for intermediaries to impose limits on the maximum number of concurrent HTTP requests that they forward, while buffering requests that exceed this limit.¶

Such intermediaries could apply a more restrictive concurrency limit to requests marked as incremental to ensure that capacity remains available for non-incremental requests, even when the maximum number of incremental requests is reached. This approach helps balance the processing of different types of requests and maintains service availability across all requests.¶

When rejecting incremental requests due to reaching the concurrency limit, intermediaries SHOULD respond with a 503 Service Unavailable error, accompanied by a connection_limit_reached Proxy-Status response header field (Section 2.3.12 of [PROXY-STATUS]).¶

For performance and efficiency reasons, a small amount of buffering might be used by intermediaries, even for incremental messages. Immediate forwarding might be exploited to cause an intermediary to waste effort on many small packets. Enabling incremental delivery might instead set limits on the number bytes that are buffered or the time that buffers are held before forwarding. Any buffering could adversely affect application latency, even if it improves efficiency. In all cases, intermediaries cannot hold data in buffers indefinitely, so data needs to be forwarded when either the time limit or the byte limit is reached.¶

5. IANA Considerations

An HTTP field named Incremental is registered in the Hypertext Transfer Protocol (HTTP) Field Name Registry, following the procedures in Section 18.4 of [HTTP]. The following values are registered:¶

Field Name:

Incremental¶

Status:

permanent¶

Structured Type:

Item¶

Reference:

This document¶

Comments:

None¶

6. References

Acknowledgments

TODO acknowledge.¶

Authors' Addresses